The proteasome is an important cellular protease that is found in two forms: the constitutive proteasome, which contains active, e.g., catalytic, subunits termed X, Y, and Z, and the immunoproteasome, which contains different active subunits termed low molecular weight proteins (LMP), LMP-2, -7, and -10. The proteasome has emerged as an important chemotherapeutic target for a number of cancers, including hematologic malignancies in general, and multiple myeloma and non-Hodgkin's lymphoma in particular. See Voorhees, P. M., et al., Clin. Cancer Res., 9: 6316-6325 (2003). Currently, the only proteasome inhibitor undergoing clinical testing is bortezomib, a non-specific proteasome inhibitor (VELCADE®, Millennium Pharmaceuticals, Inc., Cambridge, Mass., United States of America). See Adams. J., et al., Cancer Res., 59: 2615-2622 (1999). Bortezomib, a dipeptide boronic acid (see Scheme 1, below), also known as PS-341, has shown promise as an anti-neoplastic agent and is now approved by the Food and Drug Administration (FDA) for patients with relapsed/refractory multiple myeloma who have had at least two prior therapies and progressed on the last of these.

Bortezomib, however, is able to inhibit the function of both forms of the proteasome. Further, the use of bortezomib in patients typically is associated with substantial toxic effects, notably peripheral neuropathy, gastrointestinal side effects, and thrombocytopenia, which limit its clinical utility. See Orlowski, R. Z., et al., J. Clin. Oncol., 20: 4420-4427 (2002); Richardson, P. G., et al., N. Engl. J. Med., 348: 2609-2617 (2003). Thus, although bortezomib represents an important addition to the chemotherapeutic armamentarium, only 28% of patients with myeloma had partial responses or better in the phase II study that led to its FDA approval. Novel proteasome inhibitors with improved anti-tumor activity and an improved toxicity profile are needed.
While the proteasome is present in both the cytoplasm and nucleus of all cells, studies have revealed that it is not a static structure. Exposure of cells to cytokines, such as gamma-interferon, causes at least partial replacement of the three catalytic subunits X, Y, and Z, with LMP-2, -7, and -10. See Fruh, K., et al., EMBO J., 13: 3236-3244 (1994); Akiyama, K., et al., Science, 265: 1231-1234 (1994); Akiyama, K., et al., FEBS Lett., 343: 85-88 (1994); Belich. M. P., et al., Curr. Biol., 4: 769-776 (1994); and Tanaka, K., J. Leukoc. Biol., 56: 571-575 (1994). The LMP-containing proteasome has been referred to in the past as the immunoproteasome, because of initial thoughts that it might play a role generating antigens that were presented in the context of major histocompatibility class I (MHC-1) molecules as part of the immune response. See Teoh, C. Y. and Davies, K. J., Arch. Biochem. Biophys., 423: 88-96 (2004).
These proteasomes also are constitutively expressed in cell lines derived from hematopoietic precursors, and could possibly be targeted specifically by inhibitors that recognize only the LMP-type proteasome. Furthermore, such agents would spare proteasomes in gastrointestinal or neural tissues, possibly decreasing drug-related toxicities, such as those seen with non-specific agents. Additional potential applications of these agents would include their use as immune suppressive drugs to treat auto-immune and other inflammatory conditions, including graft rejection and graft-versus-host disease, or as part of vaccine therapy to specifically suppress the generation of endogenous antigenic peptides. See Wong, C., et al., J. Immunother., 21: 3240 (1998); and El-Shami, K. M., et al., Int J. Cancer, 85: 236-242 (2000). To date, however, the identification of such specific inhibitors has not yet been reported in the art.